But the dry powder of midwinter has no chance of finding a permanent resting-place on steep ice. The first strong wind that blows will remove it.
Many slopes vary from season to season and from month to month. Sometimes they are ice, sometimes snow. Such slopes, if they are ice in October, will remain ice until the following spring. May and June are the months in which the conversion of ice slopes into snow slopes usually takes place.
Glaciers in Winter
The light powder snow which is swept from rock and ice ridges is deposited on the broad glacier snow-fields, which therefore receive more than their fair share of snow. In midwinter there are few open crevasses to be seen, save at the icefalls, and most traces of concealed crevasses disappear from the upper névés. None the less, the danger from concealed crevasses is by no means trivial. The dry powder snow forms the most brittle of snow-bridges.
In the Grenz glacier accident a snow-bridge fourteen feet thick collapsed beneath a party of ski-runners. Of course, sooner or later, most crevasses are bridged, not only by powder snow, but various forms of wind-driven snow and wind-formed crust. Such crust is, of course, less brittle than powder snow-bridges, but it is quite brittle enough. The low temperature tends to increase the brittleness of snow-bridges.
In the early hours of a summer morning a snow-bridge two or three inches thick will usually be safe enough; for such bridges are generally formed of a mixture of ice and hard snow which is due to alternate melting and freezing. Such crust is much more compact, more solid, and far less brittle than crust formed by wind.
Glaciers are, in fact, safer in summer than in winter; for though there are many more crevasses open, there are far fewer dangerous crevasses whose existence cannot easily be detected. The droop on the snow and other unmistakable signs in ordinary seasons betray most concealed crevasses to the experienced eye. In winter the winds are so powerful, and their effect is so much greater (for reasons explained on pp. [446-447]), that they obliterate all traces of underlying crevasses. The surface of the snow is apparently one uniform field of wind-driven snow, and even the most expert cannot possibly detect the presence of concealed crevasses.
This raises the very vexed question of ski-ing on a rope. It is quite easy with a little practice to ski at a reasonably high speed on a rope, and to make combined swings. Roped running is, in fact, quite enjoyable, but it is not, and never can be, as enjoyable or as easy as free ski-ing. Consequently, many ski-runners will never consent to rope on the descent, save on glaciers, which are known to be badly crevassed.
There are three methods of ski-ing on a glacier:
1. A Roped Descent.—For the technique of ski-ing on a rope, see pp. [452-456].